In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot combustion gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
In the most common approach, the turbine blades are cast from nickel-base superalloys. In service, the turbine blades are subjected to extremely aggressive conditions of elevated temperature and harsh environment. It is not uncommon that some of the airfoil, particularly the portion near the tip, of the turbine blade is lost during service by a combination of erosion, corrosion, and oxidation damage. As the tip is removed, gas leakage around the turbine blade and thence around the turbine increases so that the efficiency of the gas turbine engine decreases.
Because gas turbine blades are expensive to produce as new-make articles, whenever possible the damaged turbine blades are repaired rather than scrapped. The repair involves adding new material to the tip or other damaged portion of the turbine blade by welding. In the welding operation, the same material of the turbine blade (or a different material in some cases) is melted onto the damaged area and then allowed to solidify to build up the damaged portion and return it to its permitted dimensional range.
Some of the nickel-base superalloys used in turbine blades are subject to embrittlement and cracking when the welding operation is conducted with the portion of the turbine blade adjacent to the welded region at a relatively low temperature. To accomplish the welding of these alloys, a process termed Superalloy Welding at Elevated Temperature (SWET) has been developed. As described in U.S. Pat. Nos. 5,897,801 and 6,124,568, whose disclosures are incorporated by reference, the SWET process involves preheating the portion of the turbine blade adjacent to the welding region to an elevated welding temperature prior to welding and maintaining the turbine blade at the welding temperature during the welding operation. The welding is performed in a controlled-atmosphere glove box or similar enclosure to avoid undue oxidation of the turbine blade. Before the welding operation, there may be a separate pre-welding heat treatment, and after the welding operation there may be a separate post-welding heat treatment.
The SWET welding process has been successfully applied to the weld repair of turbine blades and other superalloy components. However, the repair is relatively slow. It also requires that the welding operator control a number of different facets of the welding operation at once. Although the operators are highly skilled, performing the welding operation may overtax their abilities, and in some cases the welding cannot be accomplished successfully. Accordingly, there is a need for an improved approach to the welding of materials at elevated temperatures. The present invention fulfills this need, and further provides related advantages.